Ideally threaded fasteners should not disengage unless and until they are intentionally removed by human intervention. Unfortunately it is known that common fasteners sometimes loosen over time due to external environmental factors such as temperature cycling, high vibration, and the like. The primary method of locking a threaded fastener is by torquing a fastener having external male thread into a component having matching female thread to a predetermined level. Torquing the fastener causes a threaded shank to load in tension which in turn causes the male and female treads to tightly engage at their interface and become fixed with respect to one another due to friction between the contact faces of the thread.
Torquing the fastener to a predetermined level works well as long as there have been no mistakes in the assumed operational conditions. If the vibratory loads, thermal expansion or even the calibration of the torque wrench is different than the assumed criteria, then the fastener can loosen over time. In critical applications that involve safety such as in the aerospace industry or with high value components a redundant locking feature is necessary to ensure the integrity of the fastened joint. Various locking features have been employed over the years, but they all have one or more drawbacks in which the present invention overcomes. The use of lockwire is one example of a redundant locking feature. If implemented properly lockwire is almost 100% effective in preventing a fastener from inadvertently loosening. However, installing lockwire is extremely time consuming, tedious and if not done properly will be ineffective. Lockwire can also become cost prohibitive due to the labor intensive nature of the method.
Another common redundant locking method utilizes deformed inserts. The deformation typically includes ovalizing a portion of the insert which causes a relatively large friction load that inhibits the fastener from rotating under normal external forces. However, there are also drawbacks with locking inserts. One problem with locking inserts is that repeated use can cause the inserts to wear or yield to the point that the insert is no longer effective at locking the fastener. Another problem is that locking inserts are difficult to install properly in the first place and are even more difficult to remove and replace. The present invention addresses the problems associated with prior art fasteners by providing a novel and non-obvious solution for a reusable threaded fastener having redundant locking features.